Scherstén Anders, Elliott Tim, Hawkesworth Chris, Norman Marc
Department of Earth Sciences, University of Bristol, Will's Memorial Building, Queen's Road, Bristol BS8 1RJ, UK.
Nature. 2004 Jan 15;427(6971):234-7. doi: 10.1038/nature02221.
Osmium isotope ratios provide important constraints on the sources of ocean-island basalts, but two very different models have been put forward to explain such data. One model interprets (187)Os-enrichments in terms of a component of recycled oceanic crust within the source material. The other model infers that interaction of the mantle with the Earth's outer core produces the isotope anomalies and, as a result of coupled (186)Os-(187)Os anomalies, put time constraints on inner-core formation. Like osmium, tungsten is a siderophile ('iron-loving') element that preferentially partitioned into the Earth's core during core formation but is also 'incompatible' during mantle melting (it preferentially enters the melt phase), which makes it further depleted in the mantle. Tungsten should therefore be a sensitive tracer of core contributions in the source of mantle melts. Here we present high-precision tungsten isotope data from the same set of Hawaiian rocks used to establish the previously interpreted (186)Os-(187)Os anomalies and on selected South African rocks, which have also been proposed to contain a core contribution. None of the samples that we have analysed have a negative tungsten isotope value, as predicted from the core-contribution model. This rules out a simple core-mantle mixing scenario and suggests that the radiogenic osmium in ocean-island basalts can better be explained by the source of such basalts containing a component of recycled crust.
锇同位素比值为大洋岛玄武岩的源区提供了重要的限制条件，但已提出两种截然不同的模型来解释此类数据。一种模型认为，源区物质中富集的（187）Os是由再循环洋壳成分所致。另一种模型推断，地幔与地球外核的相互作用产生了同位素异常，并且由于（186）Os - （187）Os异常的耦合，对内核形成给出了时间限制。与锇一样，钨是一种亲铁元素（“爱铁”），在核形成过程中优先分配到地球核心，但在地幔熔融过程中也是“不相容”的（它优先进入熔体相），这使得它在地幔中进一步亏损。因此，钨应该是地幔熔体源区中核心贡献的敏感示踪剂。在此，我们展示了来自同一组用于确定先前解释的（186）Os - （187）Os异常的夏威夷岩石以及选定的南非岩石的高精度钨同位素数据，这些南非岩石也被认为含有核心贡献成分。我们分析的所有样品均没有如核心贡献模型所预测的负钨同位素值。这排除了简单的核 - 幔混合情况，并表明大洋岛玄武岩中的放射性锇可以更好地由含有再循环地壳成分的玄武岩源区来解释。
